As the need for integrated circuits for semiconductor devices having higher performance and greater functionality increases, device feature geometries continue to decrease. As device geometries become smaller, the dielectric constant of an insulating material used between conducting paths becomes an increasingly important factor in device performance.
As device dimensions shrink to less than 0.25 μm, propagation delay, cross-talk noise and power dissipation due to resistance-capacitance (RC) coupling become significant due to increased wiring capacitance, especially interline capacitance between the metal lines on the same level. These factors all depend critically on the dielectric constant of the separating insulator or inter-layer dielectric (ILD).
The use of low dielectric constant (k) materials advantageously lowers power consumption, reduces cross talk, and shortens signal delay for closely spaced conductors through the reduction of both nodal and interconnect line capacitances. Dielectric materials that exhibit low dielectric constants are critical in the development path toward faster and more power efficient microelectronics.
Alkyl silanes, alkoxy silanes and polyhedral oligomeric silsesquioxanes (POSS) and other materials comprised mainly of Si, C, O and H (SiCOH) are being evaluated aggressively for obtaining low dielectric constant (k) thin-films as interlayer dielectrics in an integrated circuit by a PECVD approach. The resulting films formed when using these precursors give dense SiCOH containing films, having dielectric constants in the range of from about 2.4 to 3.2.
Introducing porosity to the low-dielectric constant SiCOH films may serve to further lower the dielectric constant to values below 2.5.
One particular class of precursors, cyclosiloxanes, (i.e. 2,4,6,8-tetramethylcyclotetrasiloxane (TMCTS)) is being considered as a source material for the deposition of low dielectric constant (k) thin-films used as interlayer dielectrics in an integrated circuit. Cyclosiloxanes provide a thin film having an open crystal structures or cage structure (e.g. Mantz et al., “Thermolysis of Polyhedral Oligomeric Silsesquioxane (POSS) Macromers and POSS-Siloxane Copolymers”, Chem. Mater., 1996, 8, 1250–1259). PECVD of thin films from such precursors results in open areas in the structure, which leads to low packing density and hence low k values.
Chemical vapor deposition (CVD) is the thin film deposition method of choice for large-scale fabrication of microelectronic device structures, and the semiconductor manufacturing industry has extensive expertise in its use.
The purification and reproducible delivery of cyclosiloxanes for CVD is extremely critical for full-scale commercialization of the thin-film process. At present the PECVD deposition process is suffering from irreproducible delivery due to polymerization of TMCTS within the delivery lines and process hardware. Questions related to the purification of TMCTS and elimination of the polymerization must be considered. The exact polymerization mechanism is presently not known. However, studies by the inventors of the instant invention indicate that catalytic polymerization of siloxanes occurs in the presence of water/moisture, Lewis acids and Lewis bases. Accordingly, there is a need in the art to reduce water content as well as other catalytic species from siloxanes, providing improved purity, stability and utility.
There are several synthetic routes to TMCTS. For example, Takiguchi et al., report in Japanese Unexamined Patent Publication (Kokai) 50-111198, that methyl cyclic siloxanes are produced by a process in which methyl trichlorosilane reacts mildly with water.
In a still further reference (Ravi K. Laxman, Neil H. Hendricks, Barry Arkles, Terry A. Tabler “Synthesizing Low-K CVD Materials for Fab Use” Semiconductor International, Nov. 1, 2000.) TMCTS is prepared through hydrolysis of methyldichlorosilane to firstly form a linear siloxane polymer that is end-capped with trimethylsilyl groups. Alternate synthetic procedures that avoid halogenated starting materials are anticipated.
Common impurities in TMCTS and other cyclosiloxanes include water and partially halogenated or chlorinated silicon species, which could potentially form acid species in the presence of moisture. The presence of water molecules and/or acidic impurities may result in acid catalyzed polymerization mechanisms of the cyclosiloxane materials.
Accordingly, it is desired to have the appropriate cyclosiloxane material as free as possible from impurities, because, if the cyclosiloxane material contains impurities, premature polymerization in the delivery lines is possible and causes the material to no longer be considered a valid candidate for VLSI applications.
Therefore, it is an objective of the present invention to purify cyclosiloxane materials for use as CVD precursors for low dielectric constant thin films.
It is a further objective of the present invention to reduce the levels of water, basic and/or acidic catalyst molecules in a cyclosiloxane material so as to prevent or minimize premature polymerization
It is a further objective of the present invention to reduce the levels of water basic and/or acidic catalyst molecules in a cyclosiloxane material so as to prevent or minimize premature, impurity-catalyzed polymerization in a CVD reactor and associated delivery lines.
It is a still further object of the present invention to prepare low dielectric constant thin films from a cyclosiloxane precursor having reduced levels of water, basic and/or acidic impurities.